In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink the ink receptive regions accept the ink and repel the water. The ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the lithographic printing plate precursor is considered as positive-working. Conversely, if the non-exposed regions are removed, the lithographic printing plate precursor is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel ink.
Similarly, positive-working compositions can be used to form resist patterns in printed circuit board (PCB) production, thick-and-thin film circuits, resistors, capacitors, and inductors, multichip devices, integrated circuits, and active semiconductive devices.
“Laser direct imaging” methods (LDI) have been known that directly form an offset lithographic printing plate or printing circuit board using digital data from a computer, and provide numerous advantages over the previous processes using masking photographic films. There has been considerable development in this field from more efficient lasers, improved imageable compositions and components thereof.
Positive-working imageable compositions containing novolak or other phenolic polymeric binders and diazoquinone imaging components have been prevalent in the lithographic printing plate and photoresist industries for many years. Imageable compositions based on various phenolic resins and infrared radiation absorbing compounds are also well known.
Thermally imageable, single- or multi-layer lithographic printing plate precursors are also described in, for example, WO 97/39894 (Hoare et al.), WO 98/42507 (West et al.), WO 99/11458 (Ngueng et al.), U.S. Pat. Nos. 5,840,467 (Kitatani), 6,060,217 (Ngueng et al.), 6,060,218 (Van Damme et al.), 6,110,646 (Urano et al.), 6,117,623 (Kawauchi), 6,143,464 (Kawauchi), 6,294,311 (Shimazu et al.), 6,352,812 (Shimazu et al.), 6,593,055 (Shimazu et al.), 6,352,811 (Patel et al.), 6,358,669 (Savariar-Hauck et al.), and 6,528,228 (Savariar-Hauck et al.), and U.S. Patent Application Publications 2002/0081522 (Miyake et al.) and 2004/0067432 A1 (Kitson et al.).
Positive-working thermally imageable elements containing thermally-sensitive polyvinyl acetals are described in U.S. Pat. Nos. 6,255,033, 6,541,181 (both Levanon et al.), 7,399,576 (Levanon et al.), 7,544,462 (Levanon et al.), and 7,955,779 (Levanon et al.), WO 04/081662 (Memetea et al.), and U.S. Patent Application Publications 2009/0004599 (Levanon et al.), 2009/0162783 (Levanon et al.), 2009/0197052 (Levanon et al.), 2009/0291387 (Levanon et al.), and 2010/0047723 (by Levanon et al.).
Other useful positive-working lithographic printing plate precursors that can be processed using silicate-free processing solutions are described in copending and commonly assigned U.S. Ser. No. 12/948,808 (filed Nov. 11, 2010 by Levanon, Huang, and Askadsky), Ser. No. 12/948,812 (filed Nov. 11, 2010 by Levanon and Askadsky), and Ser. No. 12/948,814 (filed Nov. 11, 2010 by Levanon and Askadsky).
Offset printing plates recently have been the subject of increasing performance demands with respect to imaging sensitivity (imaging speed) and image resolution as well as resistance to common press room chemicals (chemical resistance). Often, the compositional features used to provide one desired property do not always improve other properties. While the imageable elements described in the patents, publications, and copending applications in the previous two paragraphs have provided useful advances in the art, additional improvements are still desired.
For example, for the positive-working lithographic printing plate precursors having a single imageable layer comprising a primary polymeric binder comprising recurring units of hydroxyaryl acetals or hydroxyaryl esters, the dry coating weight of the imageable layer can be greater than 1.5 g/m2 in order to achieve desired imaging and printing properties.
While these precursors provide an advance in the art, there is a desire to bring further improvements to the industry. Particularly, it is desired to improve scratch resistance of the outer surface while reducing its slipperiness so the precursors can be easily stacked, with or without interleaf papers. When lithographic printing plate precursors have too slippery a printing surface, it is difficult to stack them neatly during manufacture and packaging operations.
There is also a desire to increase what is known in the art as the “imaging window” that is also known as “imaging latitude” and is generally defined as the difference between the linearity point and clearing point. The “linearity point” is the exposure energy where the measured tonal value on a 50% halftone tint on a processed lithographic printing plate equals 50%. The “clearing point” is the minimum exposure energy required for a predetermined alkaline developer to remove the imageable layer coating in a positive-working lithographic printing plate precursor such that the revealed underlying surface is no longer sensitive to lithographic printing inks. Wider imaging latitudes, for example of 20 mJ/cm2 or more, are desired.
What can improve one of these features may not necessarily improve the others. It is a desired to find a way simultaneously to improve scratch resistance and imaging latitude while reducing slipperiness of the outermost printing surface.